1. Field of the Invention
The present invention relates to a control apparatus for a drilling machine and to a drilling method, more particularly relates to a control apparatus for a drilling machine and a drilling method applicable to drills of various diameters and especially suitable for drilling by small diameter drills.
2. Description of the Related Art
A workplece is drilled by a numerical control (NC) machine by attaching a drill to a spindle of the machine and moving it in a predetermined direction with respect to the workpiece fixed to a table while making the spindle rotate.
A "drill" is a cutting tool having a cutting edge on its tip and a flute on its body for cutting a hole. Here, FIGS. 1A and 1B show an example of a conventional method of drilling a workpiece. As shown in FIG. 1A. a center drill 102 is attached (mounted) to a spindle of a machining center of other NC machine tool. The center drill 102 is moved down to a workpiece W of a thickness H while rotating at a predetermined speed to form a prepared hole 103 in the workpLece W. This prepared hole 103 facilitates the start of the drilling of the workpLece W by the drill 104 for the main drilling.
Next., the center drill 102 is detached from the spindle. As shown in FIG. 1B, the drill 104 for the main drilling is attached to the spindle, rotated at a predetermined speed, and moved down to the workpiece W at a predetermined feed speed for cutting a hole 105 through the prepared hole 103 formed in the previous step.
At this time, the hole 105 is cut by the drill 104 by repeatedly moving the drill 104 down to the workpiece W to out it by a depth of cut S, moving the drill 104 outside of the hole 105, then again cutting by the depth of cut S. As a result of the above operation, a hole 105 is formed passing through the workpiece W of the thickness H. Note that the repeated cutting by the depth of cut S is for chip relief from the hole 105.
The above drilling method is performed by making an NC apparatus controlling the NC machine tool program execute an NC program which defines the speed of a spindle, the cutting feed speed, the depth of out S, the depth of the hole 105, and other factors.
In the above conventional drilling process, however, an NC program defining the depth of cut and the depth of the hole must be prepared for each different drilling process. Therefore, when drilling a variety of holes, troublesome work is involved and it is necessary to change the NC program to change the cutting conditions.
Note that the above drilling method was designed for forming holes having a large diameter, for example, a diameter larger than 0.5 mm. Therefore, various problems arise when applying small diameter drills of a diameter of less than 0.5 mm as they are to the above drilling method.
That is, when drilling by a small diameter drill, a small diameter drill will sometimes break when repeatedly cutting by a depth of cut S due to interference between the small diameter drill and the hole.
When drilling a through hole through a workpiece W by a small diameter drill, furthermore, the small diameter drill will sometimes easily shift from the center of the hole and break when passing through (breaking through) the workpiece W.
Furthermore, when using a small diameter drill to drill a hole through a workpiece W, the above prepared hole 103 is not formed in the workpiece W. that is, the hole is drilled by a single small diameter drill, so there is the danger of poor starting of drilling of the small diameter drill in the workpiece W.
Also, when the material of the workpiece W is a stainless steel or another heat resistant steel, the small diameter drill is heated during the cutting of the workplece W. This heat makes the small diameter drill more susceptible to breaking.
When forming a small diameter hole, therefore, it is therefore especially necessary to prepare an NC program designating the optimal cutting conditions for every drilling process.